Implantable cardiac pacemakers and other implantable devices are powered by batteries that have a finite energy capacity, such that it becomes important to monitor the status of the battery to determine remaining lifetime of the device. A typical battery, such as a lithium iodide-type battery used in implantable pacemakers has a given starting energy capacity, e.g., in the range of 600 mAh to 1800 mAh. The effective lifetime of the device is a function of the operating conditions, e.g., the parameters of the stimulus pulses and the effective output load. It is, of course, important to know when the battery is almost depleted, or empty, in order to safely replace the implanted pacemaker. As is known, it is not possible to replace only the battery, which is sealed within the case of the device, but rather the entire pacemaker must be replaced. Thus, it has been a longstanding concern of the pacemaker industry, and the implantable device industry generally, to provide an accurate indication of battery status, so as to predict when replacement may be needed.
A standard technique that has been used in pacemakers for determining the status of the battery is to measure the battery impedance. For various battery models, the characteristic of battery impedance as a function of remaining lifetime is known. Thus, the battery impedance, when it is accurately measured, provides an accurate indication of energy used, and consequently of remaining energy. A direct battery voltage measurement thus provides an actual operating parameter from which the battery status can be determined. Although not a control part of this invention, once the battery status has been determined, effective end of life (EOL) can be calculated based upon the current and projected rate of energy consumption. See U.S. Pat. No. 5,620,474, assigned to Vitatron Medical, B.V., and incorporated herein by reference.
In a typical prior art system, impedance is calculated either in response to an exterior programmer, or automatically, on the basis of a voltage difference (.DELTA.U) measured by the pacemaker. The pacemaker determines the .DELTA.U value by taking two voltage measurements across the battery, one during which the normal pacemaker circuitry is connected across the battery and the other at a time when a defined additional battery load (.DELTA.I) is applied. Based on Ohms law, the battery impedance, Z.sub.I can be calculated as EQU Z.sub.I =.DELTA.U/.DELTA.I
A condition, and consequently a problem with this straightforward approach, is that for the two measurements, battery voltage should be affected only by the additional load, .DELTA.I. However, this rarely the case. For a pacemaker, the procedure is generally to take the two measurements at the same moment in successive pacemaker cycles, in order to make a best effort to have similar normal circuit loads on the battery for the two measurements. However, the fact is that battery voltage is continuously varying because of the changing battery condition after each pacemaker cycle, e.g., by variables such as energy consumption due to delivery of a stimulus and the effective load of the lead. As discussed below, it is known that the equivalent circuit of a battery is approximately that of an RC circuit, such that directly following a delivered stimulus, the battery output voltage drops, and rises exponentially thereafter. Consequently, from cycle to cycle, there will be battery output voltage variations depending upon whether a stimulus was delivered. In particular, for dual chamber pacemakers there can be substantial variations depending upon the sequence of pacing or not pacing in the different heart chambers.
In view of the above, there is a need in the implantable battery-powered device area for a more reliable way to measure battery impedance, and thus determine battery status. The improvement must take into account, and effectively eliminate, normal variations of battery voltage and pacemaker circuit load which may occur cycle-to-cycle, in order to improve the reliability of an impedance measurement based on inserting a defined extra current load for one of the battery measurements.